Natal Origin and Spatiotemporal Distribution of Leatherback Turtle (Dermochelys coriacea) Strandings at a Foraging Hotspot in Temperate Waters of the Southwest Atlantic Ocean

Abstract

Leatherback turtles migrate long distances between nesting beaches and distant foraging areas worldwide. This study analyzes the genetic diversity, life history stage, spatiotemporal distribution, and associated threats of a foraging aggregation in the Southwest Atlantic Ocean. A total of 242 leatherbacks stranded or bycaught by artisanal fisheries were recorded from 1997 to 2021 in Uruguay, with sizes ranging from 110.0 to 170.0 cm carapace lengths, indicating that the aggregation is composed of large juveniles and adults. Results of Bayesian mixed-stock analysis show that leatherbacks come primarily from the West African rookeries, based on mitochondrial DNA sequences obtained from 59 of the turtles representing seven haplotypes, including a novel one (Dc1.7). The main threat identified in the area is the fisheries bycatch but most of the carcasses observed were badly decomposed. There was significant seasonal and interannual variability in strandings that is likely associated with the availability of prey and the intensity of the fishing effort. Taken together, these findings reinforce the importance of these South American foraging areas for leatherbacks and the need to determine regional habitat use and migratory routes across the broader Atlantic region, in order to develop effective conservation measures to mitigate threats both at nesting beaches and foraging areas.

Keywords: Río de la Plata estuary; Uruguay; conservation management and practice; genetic diversity; marine turtles; threats.

Figure 1

Map indicating the location of the Uruguay (UR) foraging ground (black star) relative to the main Atlantic leatherback nesting colonies (black triangles); BRA (Espirito Santo, Brazil); TRI (Trinidad), GUI (French Guiana and Suriname) and FWI (French West Indies including, Guadaloupe and Martinique), which represent the SECG group; FLA (Florida), DR (Dominican Republic), and ACR (Costa Rica), which represent the NWCG group; STX (St. Croix), GHA (Ghana), GAB (Gabon), and SAF (South Africa). Additional foraging ground sites (yellow circle): Espírito Santo State (ES, Brazil), SBR (Southern Brazil), and BA (Buenos Aires, Argentina) are indicated (yellow circles). Arrows represent oceanic currents: Gulf Stream (GSC), Antilles (AC), Florida (FC), South Equatorial Current (SEC), South Equatorial Counter Current (SECC), Guiana Current (GC), Brazil Current (BrC), Malvinas Current (MC), South Atlantic Current (SAC), and Benguela Current (BeC). The inset shows the Uruguayan coast. The coastal waters could be divided into three regions: Inner Estuarine Zone (IEZ), Outer Estuarine Zone (OEZ), and Oceanic Zone (OZ), which are shown as well as the Departments (CO: Colonia, SJ: San José, MO: Montevideo, CA: Canelones, MA: Maldonado; RO: Rocha).

Figure 2

Mean estimated stock contributions from Atlantic leatherback rookeries to the combined UR, BA, and SBR regional SWAO foraging aggregation using priors (weighted by rookery size) and no priors (equal contributions). Credible intervals (95%) are indicated. Abbreviations correspond with Figure 1.

Figure 3

Curved carapace length (CCL) of leatherback turtles recorded in Uruguay (n = 98). Black dashed line indicates the minimum CCL of nesting females at 130 cm from the nearest nesting colony in Brazil (adapted from Thomé et al. [15]). Red dashed line indicates the mean size of females (n = 43) and the green dashed line the mean size of males (n = 21).

Figure 4

Number of stranded leatherbacks observed during the 25 years of the study (1997–2021), grouped by seasons (n = 223). Summer in black (n = 72), Fall in orange (n = 91), Winter in grey (n = 7), and Spring in purple (n = 53).

Figure 5

Number of stranded leatherbacks observed during the 25 years of the present study (1997–2021), per month, for each zone and all the Uruguayan coasts. (A) Inner estuarine zone (IEZ), (B) outer estuarine zone (OEZ), (C) oceanic zone (OZ), and (D) all the Uruguayan coasts (note the difference in the y-axis from the other panels). The blue polygon indicates the coastal area included in each panel.

Figure 6

Stranded leatherback carcasses with evidence of human-caused interactions. (A) Artisanal set net entanglement, (B) longline fishing hook in the front flipper, (C) ropes probably used by fishers to move the turtle on an industrial fishing boat, and (D) leatherback cut in two pieces probably associated with boat collision. Photo credits: Karumbé.